The present invention relates to rotary electric motors, more particularly to permanent magnet motors comprising a plurality of rotor and stator poles having surfaces that are geometrically skewed with respect to each other.
The above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, identifies and address the need for an improved motor amenable to simplified manufacture and capable of efficient S and flexible operating characteristics. In a vehicle drive environment, for example, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability and low ripple at minimum power consumption Such a vehicle motor drive should advantageously provide ready accessibility to the various structural components for replacement of parts at a minimum of inconvenience. The above-identified copending related U.S. applications describe formation of electromagnet core segments as isolated magnetically permeable structures configured in an annular ring. With such arrangements, flux can be concentrated to provide advantageous effects as compared with prior art embodiments.
As described in the above-identified Maslov et al. applications, isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects with other electromagnet members. Operational advantages can be gained by configuring a single pole pair as an isolated electromagnet group. Magnetic path isolation of the individual pole pair from other pole groups eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched. The lack of additional poles within the group avoids any such effects within a group. Further benefits are described from utilization of three dimensional aspects of motor structure, such as a structural configuration wherein axially aligned stator poles and axially aligned rotor magnets provide highly concentrated flux density distribution in the active air gap of the machine. Such configuration provides a greater number of poles with the same individual active air gap surface areas and/or greater total active air gap surface area than conventional motors having the same air gap diameter.
In addition to benefits of flux concentration obtainable with the configurations described above, recently introduced neodymium-iron-boron (NdFeB) magnetic materials can produce larger flux densities than other permanent magnetic materials previously used in brushless machines. The use of high density producing permanent magnets in motors which comprise a great number of poles presents a concern for ameliorating undesired effects that may be introduced by cogging torque. Cogging torque is produced by magnetic attraction between the rotor mounted permanent magnets and those stator poles that are not in a selectively magnetized state. This attraction tends to move the rotor magnet to an equilibrium position in alignment with a stator pole to minimize the reluctance therebetween. As the rotor is driven to rotate by energization of the stator, the magnitude and direction of the cogging torque produced by magnet interaction with non-energized electromagnet segments changes periodically in opposition to the torque produced by the energized stator segments. In the absence of compensation, cogging torque can change direction in an abrupt manner with the rotation of the rotor. If cogging torque is of significant magnitude, it becomes a rotational impediment and source of mechanical vibration that is detrimental to the objectives of precision speed control and smooth operation.
A variety of cogging torque minimization techniques exist in the prior art. Such techniques attempt to reduce the rate of reluctance change with respect to rotor position, reduce the magnetic flux in the machine, or shift poles such that the cogging torque produced by the individual poles cancel one another. Electronic methods can be used to control the intensity of the electromagnetic interaction that takes place between permanent magnet and electromagnet surfaces. Such methods have disadvantages in that they involve complex control algorithms that are implemented simultaneously with motor control algorithms and tend to reduce the overall performance of the motor. Other alternatives would be to reduce the flux density in the magnetic circuit. Reduction of magnetic flux sacrifices advantages obtained from the newer permanent magnet materials and the flux concentration techniques of the above-identified copending applications.
Other approaches involve modifying the construction of the machine by changing the shape of the stator poles. Prior art stator poles conventionally made of stacked laminations are not readily amenable to modification. The machining processes required to effect hat is a limited range of modification of such laminated structures is complex and costly.
The need thus exists for effective cogging compensation in motors having high flux density magnitudes and concentrations that do not detract from the efficient operation and control capability of the motors, while providing practicability of cost and application.
Advantages of the present invention are achieved, at least in part, from the use of core materials such as a soft magnetically permeable medium that is amenable to formation of a variety of particularized shapes. For example, core material may be manufactured from soft magnet grades of Fe, SiFe, SiFeCo, SiFeP powder material, each of which has a unique power loss, permeability and saturation level. Core geometries and core dimensions of stator elements, with relevant tolerances, can be formed without the need to form laminations and thus can be made to optimize the magnetic potential gradient developed between coupled poles of rotor permanent magnets and stator electromagnets. By utilizing the flexibility afforded by the soft iron material to configure a variety of shapes, the magnetic potential gradient can be finely varied so that the changes in cogging torque are less abrupt A smoother operation thus can be obtained without detracting from motor control torque producing capabilities.
An advantage of the present invention is that the relationship between stator poles and rotor poles can be adjusted to control the flux distribution linking the permanent magnet elements with the non-energized electromagnet poles as a function of relative position therebetween. The structural features of one such configuration of the invention are embodied in a motor that comprises a rotor and stator each disposed in an angular ring configuration and spaced from each other by an annular radial air gap. The stator comprises a plurality of separate integral electromagnet core segments disposed coaxially about an axis of rotation. Each core segment comprises two or more poles integrally linked together. A winding is formed on the linking portion to develop, when energized with current, magnetic poles of opposite polarity in adjoining stator poles.
The stator core segments are affixed to a non-ferromagnetic support structure and distributed in the stator ring without ferromagnetic contact with each other. Thus a core segment having a non-energized winding will not have flux produced therein by energization of the winding of another, ferromagnetically isolated, core segment. The non-energized electromagnet core section, however, will be affected by the flux created by movement of a rotor permanent magnet as it approaches and passes the air gap portion facing the stator poles.
In accordance with one aspect of the present invention, the stator poles all have a common surface geometric configuration at the air gap. The rotor comprises a plurality of permanent magnets with surfaces that face the air gap, the surfaces having a common geometric configuration. The stator pole surface geometric configuration and the rotor magnet surface geometric configuration are skewed with respect to each other. Either or both of the stator pole configuration and the rotor magnet configuration may be skewed with respect to the axis of rotation. The effect of this skewing arrangement is to dampen the rate of change of cogging torque that is produced by the interaction between a rotor magnet and a pole of a non-energized stator electromagnet as the permanent magnet traverses its rotational path.
The present invention can apply these advantages to the various structural embodiments disclosed in the above-identified applications. Permanent magnets comprising high flux producing materials are oriented on the rotor surfaces with dipole polar magnetization acing the air gaps. The skewed relationship between the rotor magnet surfaces and the stator pole surfaces may be implemented to alleviate cogging torque manifestations in motors having a single axially aligned row of magnets and stator poles. In motors having a greater number of magnets and stator poles, such as those in which the stator poles of each segment are parallel to the axis of rotation, the skewed relationship effectively compensates for a potentially great cogging torque disturbance. The general structure of the latter motors are exemplified in the copending applications previously identified. Two or more axially spaced rings of separated rotor permanent magnets may be disposed circumferentially along the air gap, the number of rings being equal in number to the number of stator poles in an axially aligned stator core segment. The rows of permanent magnets are spaced axially from each other, each rotor permanent magnet being of opposite magnetic polarity to the magnetic polarity of adjacent permanent magnets in its respective ring and axial row.
Another advantage of the present invention is that the relationship between stator poles and rotor poles can be adjusted to control the flux distributions that produce cogging torque by configuring the stator pole surfaces in a geometric pattern different from the configuration of the permanent magnet surfaces. For example, the rotor magnets may be formed in rectangular surface patterns, while the stator pole surfaces may be formed in any of regular or irregular geometric patterns, the selection of a particular pattern being tailored to the flux distribution desired for a given motor application.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other add different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.